The esthetics of photovoltaic cells and modules (sometimes referred to as “PV”) have been largely ignored. This has begun to change with the development of building-integrated photovoltatic (BIPV) applications (as opposed to building-mounted PV applications. Although BIPV may make sense in a modern context, attractiveness and/or poor esthetics limit the acceptance of BIPV. In some cases, PV has encountered zoning constraints on the basis of appearance or color.
BIPV has an intrinsic advantage over building-mounted PV since the glazing and mounting structures, as well as the majority of the installation labor, is already accounted for in the cost of construction, thereby lowering the overall cost of the produced solar power. Wiring used to collect power from PV modules can be interior wiring rather than outdoor wiring, which can also reduce associated costs of the PV system. These are major advantages for building-integrated PV, as opposed to building-mounted PV.
Conventional solar modules may be produced as a laminate subassembly to which framing and interconnection elements may be added. Briefly, this laminate subassembly comprises a front glazing, an encapsulant layer, an array of cells with interconnections, a second encapsulant layer and a backsheet. The encapsulant layers are used to bond the laminate subassembly together but also serve the purpose of optically coupling the cells to the glazing. The backsheet primarily provides protection for the array of cells but is also visible through any gaps in the cell array. The various elements of the laminate subassembly may be stacked in a process commonly known as “layup”, with the entire subassembly bonded together in a press in a process commonly called “lamination”. The basic components of the layup may be simple or complex; for example, the glazing may have been treated in order to reduce its reflectance or otherwise improve its performance while the backsheet may be a polymer laminate with enhanced mechanical and moisture blocking properties. Encapsulant is commonly supplied as a film which is cut into sheets and placed onto the stack but may also be a fluid material which is applied as a coating during the layup process.
A PV module known in the art comprises active regions which are the photosensitive portions of the cells, inactive regions which comprise most of the other portions of the module (such as framing and marginal regions in and around the array of cells) and semi-active regions (such as metal interconnects and backsheet areas close to the cells) which are able to redirect a portion of incident light onto active areas by double reflection. The visual appearance of a PV module is determined by these features, with the active areas appearing relatively dark—preferably very dark if the majority of incident light is absorbed—and metallic areas appearing relatively bright. Visible portions of the backsheet depend on the material which could be transparent, particularly if glass, or white, for maximum module efficiency, or black if an overall dark appearance is desired.
A monolithic thin film PV module known in the art may be constructed in a somewhat different fashion since the cell array is generally deposited and formed directly onto the glazing which simplifies layup and lamination and eliminates one layer of encapsulant. Typically, the exposed interconnections are formed from a transparent conductor and the gaps between cells are quite small; consequently, the visual appearance is largely determined by the active area of the cells. This presents a somewhat different problem set if it is desirable to alter the visual appearance of a monolithic thin film PV module. In some other cases, the thin film cell array may have been formed on the backsheet, in which case, the encapsulant layer also optically couples the cells to the glazing. In this instance, the front contacts of the cells may be transparent or metallic.
Glass is a popular material for building facades due to its weather resistance, durability and light weight. Since it is basically transparent, a variety of methods can be used to change its appearance through the application of colorants, coatings and backdrops. Also, glass can be given variable textures during the manufacturing process at very little cost. Glass curtain wall construction is increasingly popular for commercial construction since it can withstand the weathering process. Glass curtain wall construction can integrate with window glazing in the same building skin. Compared to precast concrete, granite, marble, and other building materials, glass curtain wall construction is lighter, provides reduced wind resistance and lower precipitation loads. While sheet metal skins, in some cases, can be lighter than glass, the durability of glass is generally superior. Polymer glazing is also gaining acceptance as it can exhibit impact resistance and is typically still lower weight.
When glazing is applied in commercial construction and high rise construction it may be required to include a safety glass laminate that includes more than one layer of glazing bonded to/by an internal polymer layer. Since solar modules may have a glass surface or other glazing, they can be used as a finishing material for building facades and roofing where other hard surface materials might be used. Even though building integrated mounting may not be ideal for receiving sunlight, the incremental cost of using building integrated solar modules is low compared to applying solar panels separately since glazing, sealing and framing can be responsible for ˜40% of the module cost. When both PV glazing and architectural glazing must meet certain requirements for mechanical strength, it is desirable to optimize PV glazing for optical properties in order to increase the efficiency of the PV module. Accordingly, some design compromises may be beneficial.
Also, in locations where efficiency would be poor, dummy panels could be seamlessly integrated. Another advantage of this approach is that racking and installation costs may be avoided and wiring may be moved from outdoor exposure to an interior service chase.
BIPV may be mounted at some elevation relative to street view. In roof-top installations it can also be mounted at an incline, for example on sloped rooftops or in building awnings where panels may be mounted at an incline. The incline of the building awnings may be variable, providing shading and controlling a building's solar gain.
When glass is used for building cladding, particularly at height, much heavier glazing may be required in order to meet building codes and this may have an impact on BIPV module construction.
There have been attempts to change the appearance of solar modules. These approaches have limited efficacy or significantly reduce the module efficiency. Some attempts include:                use of multi-wire tabs (using a large number of narrow conductors instead of just two or three to interconnect cells) which dilutes the visual appearance of tabbing so that from a distance the reflective metal areas are less apparent (for example, as practiced by Day4 Energy in its “Stay-powerful™ Technology”);        use of structured tabs which recycle reflected light using total internal reflection suppressing the bright reflection of the metal making it visually indistinct (for example, Light Capturing Ribbon from Ulbrich);        use of black backsheets to make the spaces in the cell array less visible; and        use of colored backsheets.        
The ability to have a colored appearance is touted as a feature of die-sensitized solar cells. Because they are typically a thin film/liquid film, considerable graphical expression is possible. Similarly chalcopyrite cells, e.g. CIS & CIGS, can have the properties of the absorber modified to change the absorption spectrum resulting in a variety of green and earth-tone colors. Modified antireflective (AR) coatings on silicon cells are used to produce a variety of colors ranging from blue to purple, magenta, brown, gold and green.
Existing approaches can provide only very limited solutions. They can make a solar module appear almost uniformly dark or they can render color at the cost of efficiency and, in the case of organic cells, lifetime. Approaches that involve modification of the photoabsorber(s) can be problematic as they can reduce the efficiency of the cells; more importantly, modification of the photoabsorber could include a deviation in the manufacturing recipes that have been tuned for performance, and could impose a requirement for custom cells when module manufacturers tend to buy cells as a commodity. Also, since it is beneficial for all cells in a string to have matched performance, this imposes limits on the graphic possibilities. Many of the colored solar cells currently on offer are essentially attempts to turn deficiency (low efficiency) into an advantage (color).
Specific limitations of some commercially available existing approaches include:                use of multi-wire or contoured tabs merely makes the module have a more uniform appearance which is a minor benefit relative to esthetics;        use of a dark backsheet creates a significant (˜2%) efficiency penalty (since light reflected by a white back sheet is recycled) with relatively minor benefit unless a uniform black or dark blue appearance is what is desired; and        use of a colored backsheet can be used to add coloration, however, existing offerings are limited to monochrome background color and conventional cell layups. This method does not alter the appearance of exposed metal conductors.        
The above noted previously known methods for changing the appearance of PV modules may be used in combination with the methods disclosed in this application, primarily, because the previously used methods can have relatively little impact on the module appearance.
Another existing approach includes using modified absorber properties, which is generally a proposed virtue for low-performing cell technologies but is the result of limited absorption and/or light trapping. This approach may interfere with the formation of the absorber and may have a disproportionate impact on efficiency as the recipes for creating photoabsorbers may not be amenable to modification.
Still another existing approach includes using a modified antireflective (AR) coating. This approach provides a moderate repertoire of colors but sacrifices efficiency by reducing the effectiveness of the AR coat. However, using a modified AR coating may provide benefits over modifying the photoabsorber. Since the AR coating process is normally also used to passivate the front surface of the photoabsorber, this may have implications for the cell manufacturing process. Finally, this approach requires custom cells. Some cell manufacturers have used this approach successfully to produce colored cells and even cells with patterns while only sacrificing 2-4% efficiency and might be used effectively in combination with the new methods herein.
Limitations of using modified absorber properties and modified AR coatings for changing the appearance of PV modules include:
Die cells and die sensitized cells which are more typically colored than broadly absorbing may have several negative aspects, including reduced efficiency (˜4-6% for die cells versus ˜15-20% for conventional crystalline silicon PV modules) and reduced lifetime (˜2000 hours for die cells versus ˜25 years for conventional crystalline silicon PV modules). A significant detractor is that the decline of efficiency in die cells is accompanied by bleaching (loss of color). Also, known approaches may require glass/glass encapsulation, thereby increasing the module weight. Also, die cells may exhibit migration of the die over time when mounted vertically.
Coloration of chalcopyrite cells provides a limited repertoire of colors but at the cost of reduced efficiency. The ‘natural’ color of these cells tends towards dark brown which, with modifications, can be shifted towards various greens and earth tones. Since the chemistry of these cells is poorly understood and since the typical efficiencies are already significantly less than crystalline silicon modules, it is generally undesirable to modify the deposition process for cosmetic purposes. This approach requires custom cells. These known approaches may also be used in combination with the methods disclosed in this application.
The known approaches for changing the appearance of PV modules include the use of miniature cells or spheral cells in sparse arrays for skylights and glass roofs—where some transparency is required but some shading is also desirable and where color is less important. In this case, module output is reduced but not cell efficiency, thereby resulting in a low capacity cost on a $/W basis. Such modules are limited in their range of applications. This approach could however be combined with the new methods to provide ornamental solar windows.
Accordingly, there is a need for solar modules having coloration and graphics, where the solar modules are intended to be compatible with building construction and photovoltaic module manufacturing practice and without unduly impacting the module efficiency.